1. Field of the Invention
The present invention relates to a signal forming apparatus, a distance measuring device and an optical apparatus, and more particularly to a signal forming apparatus adapted for a distance measuring device for measuring a distance to a distance measuring object, a focusing device arranged to detect a state of focusing through an amount of defocus indicated by an auxiliary light reflected from a photo-taking object or the like.
2. Description of Related Art
Among distance measuring devices arranged to perform trigonometrical distance measurement by projecting a spot light onto a distance measuring object and receiving a reflected light from the distance measuring object, a device shown in FIG. 4 is popularly known. The device shown in FIG. 4 is arranged to project a spot light onto a distance measuring object 43 from an infrared light emitting diode 41 (hereinafter referred to as IRED) through a light projecting lens 42 and to receive at a position detecting element 45 (hereinafter referred to as PSD) a reflected light from the distance measuring object 43 through a light receiving lens 44. The PSD 45 is arranged to output, from its two terminals, signals A and B according to the position at which the reflected light is received. Therefore, the light receiving position of the PSD 45 can be detected by measuring these signals A and B, and then a distance to the distance measuring object 43 can be found through the light receiving position. The IRED 41 is set within the dome of a cover member which is formed in a dome-like shape.
However, the conventional distance measuring device shown in FIG. 4 has the following problems. In respect of S/N, for a feeble signal, noises generated by an amplifier of a signal processing circuit and by the resistance of the PSD 45 are added to every synchronous integral signal. Therefore, in order to obtain a signal component in a larger amount, it is necessary to increase the size of a distance measuring block which is composed of the light projecting lens 42, the light receiving lens 44, etc., and also to increase the light projecting power of the IRED 41 at the expense of possibility of reduction in size of the distance measuring device.
Further, it is necessary to increase the length of the PSD 45 for a wider distance measuring range. With the PSD 45 arranged to be longer for a wider distance measuring range, however, the varying rate of the signals A and B in relation to distances becomes smaller to deteriorate the accuracy of position detection.
Among the known distance measuring devices of the kind making trigonometrical distance measurement by projecting a spot light onto a distance measuring object and receiving a reflected light from the object, some of them are arranged to use a pair of sensor arrays as a light receiving element, to form an image of light reflected by the distance measuring object on each of the sensor arrays and to compute a distance to the distance measuring object by carrying out a correlative arithmetic operation to obtain a phase difference between the pair of images of reflected light received. Such an arrangement was disclosed, for example, in Japanese Patent Publication No. HEI 5-22843 and Japanese Laid-Open Patent Application No. HEI 9-42955. In the case of such a phase-difference detecting type distance measuring device, the so-called active AF (automatic focusing) method can be carried out to detect the light receiving position at a higher rate of resolution by virtue of the use of the multi-divided sensor array than in the case of carrying out the active AF method using the PSD. Besides, the active AF method using the multi-divided sensor array, such as a CCD or the like, has a better S/N than the active AF method using the PSD, because the active AF method using the multi-divided sensor array is almost completely unaffected by thermal noise caused by the resistance of an output part which becomes a dominant source of noise in the case of the active AF method using the PSD.
In the active AF method using the PSD, the AF action is performed with a distance computed by detecting the barycenter of the received light image. Therefore, the IRED used by this method can be arranged to have only one light emitting part at each light projecting element for one distance measuring point. FIG. 5 shows by way of example the pattern of light emission of an IRED adapted for multi-point distance measurement to be used by the active AF method using the PSD.
Referring to FIG. 5, the IRED having the light emitting pattern has three light projecting elements which respectively correspond to distance measuring points in three directions. One light emitting part is provided for each of the three light projecting elements which are arranged for distance measuring points in three directions.
According to the phase-difference detecting type active AF method, on the other hand, a plurality of light emitting parts are arranged for each of the light projecting elements of the IRED and the sensor arrays are arranged to output signals with many edges in a direction perpendicular to the pixel columns of the sensor arrays. It is known that the accuracy of the distance measurement increases accordingly as the number of these edges increases. FIGS. 6(a) to 6(c) and FIGS. 7(a) to 7(c) show the details of this method.
FIGS. 6(a) and 7(a) show patterns of light emission of the IRED arranged for distance measuring points in five directions. FIG. 6(a) shows a case where one light emitting part of the vertically extending bar-like shape (shown in black) is provided for each light projecting element corresponding to one distance measuring point. FIG. 7(a) shows another case where two light emitting parts of the vertically extending bar-like shape (shown in black) are provided for each light projecting element corresponding to one distance measuring point.
FIGS. 6(b) and 7(b) show images respectively formed by the light emission patterns of FIGS. 6(a) and 7(a) on each sensor array which corresponds to one distance measuring point. FIGS. 6(c) and 7(c) respectively show output signals of the sensor arrays to be used for correlative arithmetic operations by aligning the output values of the pixels of the sensor arrays. A difference between the output values of adjacent pixels increases at two parts, i.e., at the rise and fall of the output signal, as shown in FIG. 6(c). In the case of the output signal shown in FIG. 7(c), on the other hand, the difference between the output values of adjacent pixels increases at four parts. In carrying out the correlative arithmetic operation for detecting a phase difference, the larger the number of parts where the difference between the output values of adjacent pixel becomes large, the lesser the adverse effect of the state of reflected light or external noises on the distance measuring accuracy. Therefore, assuming that the light emission patterns shown in FIGS. 6(a) and 7(a) are obtained under the same conditions, such as the area of light emission, an IRED driving current, etc., distance measuring accuracy of a distance measuring device using the IRED of the light emission pattern of FIG. 7(a) is better than that of a distance measuring device using the IRED of the light emission pattern of FIG. 6(a). In view of this, many of the distance measuring devices of the above-stated phase-difference detecting type active AF method are arranged either to use an IRED having a plurality of light emitting parts at each light projecting element corresponding one distance measuring point or to use an IRED of the light emission pattern in which many edges are generated in the output signals of sensor arrays in the direction perpendicular to the column of pixels.
However, the conventional distance measuring devices of the phase-difference detecting type active AF method described above have the following problem.
As mentioned above, the larger the number of the bar-shaped light emitting parts of each light projecting element of the IRED, the better the accuracy of distance measurement. In actuality, however, the number of the bar-shaped light emitting parts is determined under various restrictions, as follows.
For example, since the size of the distance measuring device would increase if each light projecting element for one distance measuring point is arranged in one package (within a dome) separately from other light projecting elements, it is generally practiced to arrange the light projecting elements for all distance measuring points in one package. However, the total width of a chip forming the IRED is limited by the allowable size of the IRED package.
Further, in forming the light emitting parts for each light projecting element of the IRED, the rate of machining precision also imposes a limit on the extent to which the width of each bar-shaped light emitting part can be reduced. The feasible number of the bar-shaped light emitting parts is thus limited also by this limitation in addition to the limitation imposed on the total width of the chip.